You also managed to describe how the Roman Empire worked.
How is this different than what the Romans did?
Besides, if you are a Megalomaniac who wants to take over the world, don't reinvent the wheel, use techniques that already work.
The Romans had more infrastructure and operated from centralized locations. The Legion, like the Mongols, are more of a marauding horde. The Mongol Empire didn't have a real capital until after Genghis Khan died and Ogedei built Kharkorum.
The Legion, like the Mongols, used fear to a much greater extent against their enemies. The Mongols spared artisans of conquered enemies so that they could write about the Mongols' brutality to instill fear in others.
The Mongols were rather fond of the kinds of tests that the Legion gave to Nipton. The Mongols liked to test the loyalty of conquered cities by randomly killing people and seeing if anyone resisted.
The Legion's use and treatment of slaves is probably closer to the Mongols' system of slavery than the Romans'.